A critical event in the immunological response is initial activation of an effector T cell when it recognizes an antigen in the context of self- major histocompatibility complex molecules on a cell surface. The process of extending from T cell antigen recognition to activation has been characterized primarily through use of immune T cell populations. These studies characterize what might be called antigenicity; that is, the ability of previously stimulated cells to recognize foreign antigen. However, vaccine design, transplantation, and autoimmunity all require that we understand immunogenicity, the ability to activate naive resting T cells, in addition to antigenicity. This UCLA Symposium will focus on all aspects of immunogenicity; although this is a central question in immunology, it has never been the focus of a conference. The plenary sessions, workshops, and poster session have been constructed around five themes: the ligand, the antigen presenting cell, the response, regulation of the immune response, and the organism. Within this framework we hope to encourage many lively small group discussions, as well as present plenary sessions and special lectures of interest to the entire group.